Electric discharge tube containing an alkaline metal



M 21, 1952 A w. H. L. CLAESSEN 2315,14

ELECTRIC DISCHARGE TUBE CONTAINING AN ALKALINE METAL Filed Aug. 17, 1949 INVENTOR. Wilhelmus Hubertus Louis Cloessen AGENT characteristic curve, shifts.

v the original adjustment. phenomenon may occur particularly in so-called Patented Oct. 21, 1952 UNITED STATES PATENT orrics ELECTRIC DISCHARGE TUBE CONTAINING AN ALKALINE METAL Wilhelmus Hubertus Louis Claessen, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Com trustee pany, Hartford, Conn., as

4 Claims.

more particularly to cathenergy for incandescence.

It has been found that in tubes of the kind comprising a cathode coated with barium oxide the phenomenon may occur that during the life of the tube the Ia-Vg or, in a diode the Ice-Va Thus, for example, it frequently occurs that during operation the Ia-Vg characteristic curve of a tube comprising one or more grid electrodes shifts to the region of higher positive grid voltages, so that the negative grid voltage must be reduced to re-establish This inconvenient battery tubes, i. e. in tubes having a cathode of the directly heated type, more particularly if the cathode is adapted to be operated with low heating energy. I

; Researchrevealed that the said phenomenon is due to the fact that emissive material, particularly barium, which has reached the controlelectrode or, in a. diode, the anode, for example, during the formation of the cathode and/or during thefixing of the getter is liable gradually to disappear during operation, since it associates With subsequently emanating gases. This results in the work potential of the said electrode being increased, so that the voltage applied must be raised by the same amount to re-establish the initial adjustment.

The phenomenon is more pronounced in tubes comprising a cathode of the directly heated type requiring'low heating energy than in tubes comprising a cathode of the indirectly heated type and this is due to the fact that during operation of an indirectly heated-cathode apparently sufficient barium is volatilized to balance the loss of barium from the electrode referred to, so that the shift of the characteristic curve does not occur to a serious extent. However, the said phenomenon may be very serious in battery tubes particularly in diodes, since for a satisfactory operation of a detector diode it is desirable that the current flow should start when the anode has a low negative voltage relatively to the cathode. In consequence of the aforesaid phenomenon the Ia-Va-characteristio curve, during life shifts to positive values of Va, so that the bend at the beginning of the diode current characteristic curve shifts to the positive range and thus results in distortion of the low-frequency signal.

According to the invention, the said difficulties can be mitigated if in a high vacuum discharge tube having an electrode system comprising at least a cathode containing barium oxide and an anode one or more alkaline metals are available in a free state. 7

In general it is sufficient, if one of these metals is available in the form of vapour, it being possible for the vapour tension to be lower than the tension of saturated Vapour. In this case the cold electrodes are constantly struck by atoms of the said metal, so that the atomic layer of this metal on the electrode surface is maintained. Highly satisfactory results in this respect have been obtained with caesium. However, as an alternative the other alkaline metals lithium, potassium, sodium or rubidium may be used. These metals have a sufliciently high vapour tension to maintain the said layer on the cold electrodes, whilst at the same time the work potential is suificiently understood and readily carried'into effect, one

example will now be described with reference to the accompanying drawing in which:

Fig. 1 shows two IaVg curves showing the displacement eifect on ageing referred to above;

Fig. 2 shows two IaVa curves showing the displacement lfect in the case of a diode and.

Fig. 3 shows a tube according to the invention, the electrodes being shown in .elevation and the envelope in section. V

Fig. 1 shows the Ia-Vg-characteristic curve of a tube comprising acontrol grid. The working point A is obtained by applying a bias voltage of -25 v. to the control grid. In consequence of the disappearance of the barium film from the control grid, the characteristic curve will shift to the right during the life of the tube, so that finally point B becomes the working point. In order to re-establish the initial adjustment it will be necesssary to apply to the grid a voltage less negative by, say, 0.3 v., for example a voltage of about -2.2 v. Obviously, it is highly inconvenient, if the adjustment of the voltages in an apparatus must be varied during the life of the tube. If in a tube according to the invention caesium is available in a free state, the work potential of the grid is influenced thereby in such manner that the characteristic curve shifts by approximately 0.5 v. to higher negative values of the grid voltage. Owing to the comparatively high vapour tension of caesium the surface layer which, as a rule, is atomic, is maintained even if part of the caesium is converted by emanating gases into caesium compounds. Even a variation of the supply of barium on the grid surface does therefore not result in an appreciable shift of the characteristic curve.

Variation of the work potential in the gases of a diode is very harmful. In order to ensure undistorted detection of signals of high. modulation percentage it is necessary that the bendof the diode characteristic curve (Fig. 2, line I), should lie in the negative anode voltage range that is to say the diode allows current to pass even at a low negative anode voltage. In practice it is required that an anode current of 0.3 ya. (that is the beginning of current flow) passes at a negative anode voltage as low as 0.1 to 0.3 volts. However, if the anode surface is made of pure nickel, a current flow of 0.3 11.3,. (Fig. 2, line ID will not be obtained until a positive anode voltage of 0.2 v. This means that the negative peaks of the modulation voltages lie on the bend of the Ia-Va characteristic curve and may thus be distorted or even cut off. If in these circumstances the anode surface is coated with caesium, current will start to flow at as low as approximately 0.3 v. of negative external anode potential, detection taking place in accordance with the straight part of the Ia-Va.-characteristic curve (line I).

Fig. 3 shows a tube having a cathode of the directly heated type base. 2 in which contact pins are sealed, a pentode system 3'and. a diode system 4.. Secured in the tube is a plate I on which are arranged two tubes 5 containing barium and magnesium and a tube 6 containing a caesium compound.

On heating, firstly Cs volatilizes and then. Ba and Mg volatilize at a higher temperature. Owing to the small supply of CS and the comparatively high vapour tension, the Cs remains in the tube in the form of vapour, so that the monatomic Cs-film on the surface of the cold electrodes can be maintained withouta Cs-mirror being formed on the anode. wall. The supply of Cs may even be so small that it remains entirely inthe form of vapour except for the atomic film formed on the cold surfaces. However,- in such a layer the Cs is no longer available in the form of free Cs but is strongly adsorbed at the surface, so that it is possible for the tension of the Cs vapour in the tube to be lower than the tension of saturated vapour. Inthis tube the advantages described with reference to the characteristic curves of Figs. 1 and 2 are thus obtained in the pentode and diode systems respectively, the incandescent cathode of directly heated type of the tube being constituted by awire 8a thickand comprising a bulb I, a-

ness and consuming a heating energy of materially less than 03 watt (1.4 volt, 0.025 amp). A thin filament of this kind is not capable of maintaining a barium film on the surface of the cold electrodes by volatilization of barium, particularly when such a cathode has a very low operating temperature owing to improvements in the emissive layer.

What I claim is:

1'. An electric discharge tube having a given anode current characteristic curve, comprising van envelope, a barium oxide coated cathode source of electron emission Within said envelope,

an anode of given work potential spaced from said cathode within said envelope, and an alkali metal in said tube in the free state in an amount only suflicient to maintain said given anode work potential substantially constant during the operating life of said tube.

2. An electric discharge tube having a given anode current characteristic curve, comprising an envelope, a barium oxide coated cathode source of electron emission within said envelope, an anode of given work potential spaced. from said cathode within said envelope, and caesium metal in said tube in. the free state in an amount only sufficient to maintain said given anode work potential substantially constant during the operating life of said tube.

3. An electric. discharge tube having a given anode current characteristic curve, comprising an envelope, a filamentary type barium oxide coated cathode source of electron emission within said envelope, an anode of given work potential spaced from said cathode within said envelope, and caesium metal in said tube in the free state in 'an amount sufiicient only to maintain the said given anode work potential substantially constant during the operating 1ife of said tube.

4. An electric discharge tube having a given anode current characteristic curve, compirsing an envelope, a barium oxide coated directly'heatedcathode source of electron emission within said" envelope, an anode of given work potential spaced from said cathode within said envelope,

' and an alkali metal insaid tube in the free state in an. amount sufficient only to maintain the' said given anode work potential substantially constant during the operating life of said tube.

' NILHELMUS HUBERTUS LOUIS CLAESSEN.

REFERENCES CITED The following references are. of record. in the file of this .patent:

UNITED STATES PATENTS Number Name Date 1 ,892,244 Langmuir et al Dec. 27, 1932 1,961,750 Ewest June 5, 1934 2,060,552 Braselton' Nov. 10, 1936 

1. AN ELECTRIC DISCHARGE TUBE HAVING A GIVEN ANODE CURRENT CHARACTERISTIC CURVE, COMPRISING AN ENVELOPE, A BARIUM OXIDE COATED CATHODE SOURCE OF ELECTRON EMISSION WITHIN SAID ENVELOPE, AN ANODE OF GIVEN WORK POTENTIAL SPACED FROM SAID CATHODE WITHIN SAID ENVELOPE, AND AN ALKALI METAL IN SAID TUBE IN THE FREE STATE IN AN AMOUNT ONLY SUFFICIENT TO MAINTAIN SAID GIVEN ANODE WORK POTENTIAL SUBSTANTIALLY CONSTANT DURING THE OPERATING LIFE OF SAID TUBE. 